1. Field of the Invention
The present invention relates to a method and an apparatus for supplying a granular raw material for reduced iron, whose main components are carbonaceous material as reductant and iron oxide, onto a hearth and, in particular, to a method and an apparatus for supplying a granular raw material for reduced iron in which the supply amount of such a material is adjusted or a leveling method and a leveler for dispersing and leveling such a material over an entire hearth when supplying the material to a reduction furnace for reduction.
2. Description of the Related Art
Conventionally, when supplying a pellet-like or briquette-like granular raw material for reduced iron, whose main components are carbonaceous material as reductant and iron oxide, to a movable-hearth furnace, the granular raw material for reduced iron, which is obtained through granulation by a granulating machine, is dried by a drying furnace, and then supplied onto the hearth of the movable-hearth furnace by conveying it by a supplying apparatus, such as a belt conveyor or an oscillating conveyor. To adjust the supply amount, the conveying speed of the belt conveyor or the frequency of the oscillating conveyor is adjusted.
A means for supplying a granular raw material for reduced iron other than belt conveyors or oscillating conveyors is disclosed, for example, in U.S. Pat. No. 5,885,521. As shown in FIG. 8, which is a schematic side view of a supplying apparatus, showing a material to be supplied and a pellet leveler, in the supply means as disclosed in the above-mentioned U.S. patent, a granular raw material for reduced iron dried by a drying furnace is supplied to a hearth 36 of a rotary hearth furnace, which is a movable-hearth furnace, through a supply pipe 102, which is perpendicular to the hearth. By means of a pellet leveler 104 (adjustable gate) provided on a side surface of the forward end portion of the pipe 102, which serves as a leveling means, the gap of the discharge outlet is adjusted.
In the drawing, numeral 112 indicates a leveler, serving as a leveling means like the leveler 104. The leveler 112 levels the granular raw material for reduced iron to a single or double layer depth.
The supply amount adjusting means which adjusts the amount of granular raw material for reduced iron supplied onto the hearth by adjusting the conveying speed of the belt conveyor or the frequency of the oscillating conveyor, has the following problems to be solved. During normal operation, the amount of the material discharged from the granulating machine and the drier fluctuates greatly, so that it is difficult to control with high accuracy the amount of the material supplied onto the hearth of the movable-hearth furnace. When the supply amount fluctuates, the operating conditions for the furnace fluctuates due to variation in the heat load of the burner. Further, the number of layers of the material laid on the hearth fluctuates, and the material heating condition varies, resulting in a deterioration in the quality of the reduced iron to be obtained as the product.
When an oscillating conveyor is used for conveying the granular raw material for reduced iron, the material will be broken or pulverized due to the oscillation during the conveyance or the shock as a result of falling onto the hearth. This makes it impossible for reduction to be effected to a sufficient degree, resulting in a deterioration in the quality of the granular reduced iron obtained as the product. Further, a part of the powder generated will be scattered in the flue gas as a secondary dust, resulting in a reduction in yield. Further it is to be noted that, for effective utilization of the secondary dust, it is desirable for the amount of metal, such as zinc or lead, which is evaporated from the material as a result of the heating and reduction in the furnace, to be large. However, since the powder of the material is mixed with the secondary dust, the value of the secondary dust deteriorates.
When the movable-hearth furnace is a rotary hearth furnace, the supplied granular raw material for reduced iron goes around inside the furnace before it is discharged to the exterior of the furnace as granular reduced iron, so that the discharge position for the granular reduced iron is close to the supply position for the material. Of course, the interior of the furnace must be maintained at high temperature by combustion gas or the like, and negative pressure is maintained in the interior of the furnace so that the high-temperature gas or unburned gas may not leak to the exterior of the furnace. However, when a belt conveyor or an oscillating conveyor is used as the supply amount adjusting means, a large gap must be provided at the supply inlet for the material. Thus, outside air is allowed to enter through this gap, and the granular reduced iron undergoes oxidation again, resulting in a deterioration in the quality of the granular reduced iron.
On the other hand, when the above-mentioned pellet leveler, provided on a side surface of the forward end portion of the material supply means, is used as the supply amount adjusting means, the above problem experienced with the belt conveyor or the oscillating conveyor may be mitigated to some degree. However, it cannot be called satisfactory since it does not help to adjust the supply amount of the granular raw material for reduced iron with high accuracy. That is, although the gap can be adjusted, it is not possible to adjust the gap during operation, and the amount of the material supplied onto the hearth is adversely affected, for the fluctuation in the amount of material discharged from the drier and the fluctuation in the average size of the material cannot be coped with. When the pellet leveler is provided at a fixed position, without undergoing vertical fine adjustment, the pellet leveler is mounted in such a way as to hinder a part of the flow of the material, so that the thinner the material layer on the hearth, the larger the degree to which the material collapses, resulting in an increase in the amount of material in the form of powder or pieces.
Further, a granular raw material for reduced iron contains oily components, such as tar or oil as in the case of coal or ironworks dust, and the oily component adheres to the inner wall of the supply pipe, which is at high temperature due to radiation heat from the hearth, and grows gradually. As a result, the discharge of the material from the pipe is hindered, and the pipe is clogged in the end, making it impossible to adjust the supply amount of the material with high accuracy.
Accordingly, it is a first object of the present invention to provide a supply amount adjusting method and a supply device for a granular raw material for reduced iron in which it is possible to prevent intrusion of outside air into the furnace and breaking or pulverization of the granular raw material for reduced iron and in which the gap is adjust from outside the movable hearth furnace, thereby making it possible to adjust with high accuracy the amount of granular raw material for reduced iron supplied onto the hearth of the furnace.
In the supply amount adjusting method for a granular raw material for reduced iron of the present invention, when feeding the granular raw material for reduced iron into the material receiving hopper and supplying it onto the hearth of the movable hearth furnace from the material receiving hopper through a material supply means, preferably, a cylindrical member, the material supply means is moved vertically to thereby adjust the gap between the lower end of the material supply device and the hearth.
In this case, the fluctuation in the amount of granular raw material for reduced iron fed into the material receiving hopper is absorbed, and the interior of the material supply means is maintained in a condition in which it is filled with the granular raw material for reduced iron, that is, substantially in an enclosed state, so that it is possible to reduce the amount of outside air allowed to enter the movable hearth furnace. Further, the gap between the lower end of the material supply means and the hearth can be adjusted, whereby it is possible to adjust with high accuracy the amount of granular raw material for reduced iron supplied onto the hearth of the movable hearth furnace. Further, as stated above, the interior of the supply means is filled with granular raw material for reduced iron, so that it is possible to reduce the speed at which the granular raw material for reduced iron is dropped onto the hearth.
It is desirable to cool the above-mentioned cylindrical member.
When the material supply means is at high temperature, the oily component contained in the granular raw material for reduced iron adheres to the inner wall of the cylindrical member serving as the material supply means, so that the discharge of the granular raw material for reduced iron from the cylindrical member is gradually hindered, and there is the danger of the member being eventually clogged. However, since the cylindrical member is cooled, no oily component adheres to the inner wall of the cylindrical member.
The supply device for a granular raw material for reduced iron of the present invention for realizing the above supply amount adjusting method, comprises a material receiving hopper for receiving the granular raw material for reduced iron, a material supply means for supplying the granular raw material for reduced iron received by the material receiving hopper onto the hearth of a movable hearth furnace, and a gap adjusting means for vertically moving the material supply means to thereby adjust the gap between the lower end of the material supply means and the hearth.
In the above-described supply device for a granular raw material for reduced iron, it is further desirable to provide a cooling means for cooling the material supply means, preferably the cylindrical member.
Next, a second object of the present invention is to provide a method and an apparatus for leveling a granular raw material for reduced iron which do not adversely affect the productivity of the granular reduced iron, and in which there is no fear of the granular raw material for reduced iron being broken or pulverized, making it possible to reliably disperse and level the granular raw material for reduced iron supplied to the movable hearth furnace over the entire surface of the hearth.
In the method of leveling a granular raw material for reduced iron of the present invention, the granular raw material for reduced iron is supplied onto the hearth of a movable hearth furnace through a material supply means, and the granular raw material for reduced iron on the hearth is moved in the hearth width direction by rotating a spiral vane on the downstream side of the material discharge outlet for discharging the granular raw material for reduced iron from the material supply means with respect to the hearth moving direction, dispersing and leveling the material on the hearth surface.
In this case, the granular raw material for reduced iron on the hearth is moved in the hearth width direction by a rotating spiral vane, whereby the granular raw material for reduced iron on the hearth can be dispersed in the hearth width direction.
It is desirable to control the apparatus such that when the amount of the granular raw material for reduced iron supplied from the material supply means is increased or the average diameter thereof is increased, the distance between the hearth and the spiral vane is increased, and that when the supply amount is reduced or the average diameter of the material is reduced, the distance between the hearth and the spiral vane is reduced.
In this case, it is possible to disperse the granular raw material for reduced iron in the hearth width direction without applying an excessive crushing force thereto.
It is desirable to move the granular raw material for reduced iron supplied from the material supply means toward both ends from a position near the center with respect to the hearth width direction by means of the spiral vane.
It is desirable to move the granular raw material for reduced iron supplied from the material supply means toward both ends with respect to the hearth width direction to disperse it, using the material supply means or a position near the center of the material discharge outlet as a movement junction.
Further, it is desirable to cool the rotation shaft around which the spiral vane is provided.
An apparatus for realizing the method for leveling a granular raw material for reduced iron comprises a material supply means for supplying a granular raw material for reduced iron onto a hearth of a movable hearth furnace, and a granular raw material for reduced iron leveling means arranged on the downstream side of a material discharge outlet through which the granular raw material for reduced iron is discharged from the material supply means with respect to a hearth moving direction and adapted to disperse and level the granular raw material for reduced iron supplied onto the hearth over the upper surface of the hearth, wherein the leveling means comprises a rotation shaft rotated in the movable hearth furnace, a leveling member provided in the outer periphery of the rotation shaft and consisting of a spiral vane for moving the granular raw material for reduced iron on the hearth in the hearth width direction, and a rotating device for rotating the rotation shaft.
It is desirable for the rotation shaft to be supported so as to be vertically movable by a shaft raising/lowering device.
In this case, it is possible to raise or lower the shaft in accordance with the increase/decrease speed of the supply amount of the granular raw material for reduced iron or the variation speed of the average diameter of the material, whereby it is possible to disperse the material in the hearth width direction without applying an excessive crushing force to the granular raw material for reduced iron.
It is desirable for the spiral vane to be composed of a left-turn vane and a right-turn vane.
In this case, it is possible to move the granular raw material for reduced iron supplied from the material supply means toward both ends from a position near the center of the hearth with respect to the hearth width direction to disperse the material.
It is desirable for the left-turn vane and the right-turn vane of the spiral vane to be separated at the material supply means or at a position near the center of the material discharge outlet.
In this case, the reduced iron material supplied from the material supply means can be moved to both ends with respect to the hearth width direction to disperse the material, using the material supply means or a position near the center of the material discharge outlet as a movement junction.
Further, it is desirable to provide the leveling member with a cooling means for cooling the rotation shaft.
In this case, the thermal expansion of the rotation shaft of the cooling member is restrained by cooling, and the rotation of the leveling member is not hindered, so that it is possible to continue to cause the leveling member to rotate smoothly during the operation of reducing the granular raw material for reduced iron.
Further, by applying the leveling method and leveling apparatus according to the second aspect of the invention to the method and apparatus for supplying a granular raw material for reduced iron according to the first aspect of the invention, it is possible to advantageously achieve a synergistic effect.